When females of the DDK inbred mouse strain are mated to males of many other inbred strains, the vast majority of the resulting embryos die during preimplantation development. The reciprocal matings, between DDK males and females of other inbred strains are fully viable. The maternal and paternal components of this lethal trait segregate as two closely-linked loci. Over the last three years, we have succeeded in localizing these genes to a small area (approximately 300 kilobases) of mouse chromosome 11. The isolation of these genes should allow a determination of how the maternal and paternal genes interact to result in the death of preimplantation embryos and, perhaps, how this interaction results in the preferential segregation of chromosomes 11 (meiotic drive) among heterozygous females. The isolation of these genes will also allow an investigation of how the non-Mendelian, parental-origin dependent, transmission of this trait is achieved.